Photoconductive control circuit for light amplifiers and like device



Oct. 25, 1960 B. KAZAN 2,957,991

PHOTOCONDUCTIVE CONTROL cmcun FOR LIGHT I AMPLIFIERS AND LIKE DEVICE Filed Sept. 30, 1957 2161 Mai MP I IN VEN TOR. BENJAMIN KAzAn Arm/Mir United States Patent M PHOTOCONDUCTIVE CONTROL CIRCUIT FOR LIGHT AMPLIFIERS AND LIKE DEVICE Benjamin Kazan, Princeton, N..I., assignor to Radio Corporation of America, a corporation of Delaware Filed Sept. 30, 1957, Ser. No. 686,949 1 5 Claims. (Cl. 250-213) v This invention relates to photoconductor circuits, and more particularly to circuit means for controlling the conductivity of photoconductor materials.

Photoconductor materials, that is, materials that have a conductivity which varies with illumination by radiant energy, have a variety of uses. As an example, they may be used to control the operation of a relay or to give an indication on a light meter. Also, electrical energy may be converted into light images by using photoconductive materials and materials that utilize the property of electroluminescence. In such devices an electric field may be applied across a series combination of a photoconductive material in close proximity to an electroluminescent material constructed in the form of a fiat panel. If the photoconductive material is irradiated by modulated radiant energy, light will be emitted from the electroluminescent material which varies from point to point in intensity in accordance with the modulating signal.

One type of light amplifying device utilizes means to apply from a single source of alternating voltage a pulsating direct voltage to the photoconductive material and an alternating voltage to the electroluminescent material. This is accomplished by a direct voltage bias on the photoconductive material or by the use of rectifiers. The pulsating direct voltage increases the sensitivity of the photoconductor material, thus providing a more eificient light amplifier device.

In a light amplifier device the photoconductive material is usually shielded from the light that is emitted from the electroluminescent material. A second type of light amplifying device, called a regenerative storage light amplifier, allows the light from the electroluminescent material to excite the photoconductive material, thus, once the device is activated by light the light emitted by the electroluminescent material Will feed back to the photoconductor and retain Ior store the image even when the exciting radiation is removed;

It has been found, however, that the image produced byra light amplifier without feed back will remain on the device for a considerable period of time after the radiant excitation is removed. It may take many seconds for the image to decay and even hours after removal of the radiant excitation to the device a new incident light may re-excite the old image. This is caused by the fact that the conductivity of the photoconductor material does not return immediately to its un-excited value after radiant excitation is removed. In the storage device, of course,

Patented Oct. 25, 1960 ductivity of a photoconductor material after radiant excitation is removed from the material.

It is another object of this invention to provide a simple erasing means to rapidly remove the image from a light amplifier or storage device.

It is yet another object of this invention to provide an improved means that is both simple and inexpensive for rapidly erasing the picture information from a light amplifier panel or storage device panel and conditioning the panel for the immediate reception of a new image.

In accordance with the invention, the rate of decay of the conductivity of "a photoconductor material is varied after radiant excitation is removed therefrom by altering the character and value of the electric field applied to the material. In accordance with one form of the invention the image formed by a light amplifier or regenerative storage light amplifier is erased by the reversal of the polarity of the operating voltages that are applied to the amplifier or storage device to reduce the current flow through the elements of the device to their unexcited states.

The invention will be further understood when the following description is read with reference to the accompany-ing drawings in which:

Figure 1 is a simplified circuit diagram of a load circuit controlled by a photoconductor material constructed in accordance with the invention;

Figure 2 is a schematic circuit diagram of a light amplifier panel and associated circuits, constructed in accordance with the invention;

Figure 3 is a schematic circuit diagram of a regenerative light storage circuit illustrating another embodiment of the invention; and

Figure 4 is a circuit diagram of a light amplifier circuit illustrating another form of the invention.

Referring now to the drawings, and in particular to Figure 1, an elementary form of a photoconductor control circuit includes a photoconductive element or photoconductor 10 connected in series with a load circuit 12. The load circuit may be a relay, meter, electroluminescent cell, or other desired device. The photoconductive material may be made of a cadmium sulfide (CdS) powder in a suitable binder. A source of energizing potential, here illustrated as a battery 14, is connected across the photoconductor element 10 and the load circuit 12 through a polarity reversing, double-pole, double-throw switch 16. The photoconductor 10 and the load circuit 12 are connected between the common contact 18, 18' of the switch 16 and the battery 14 is connected to a first set of contacts 20, 20 on the switch. A second set of contacts 22, 22' are cross-connected to the first set of contacts 20, 20' so that when the movable elements 24, 24 of the switch are connected from the first set of contacts 20, 20' to the second set of contacts 22, 22 the polarity of the voltage applied to the photoconductor 10 is reversed.

Assume that the movable elements 24, 24' are connected to the second set of contacts 22, 22'. The positive terminal of the battery 14 is connected to the photoconduct-or element 10 and the negative terminal is connected to the load circuit 12. If the photoconductor 10 is subject to a radiant excitation, such as by light, a current will flow. If the excitation is removed, the current begins to decay through the photoconductor but remains at a substantial value for a considerable period after the excitation is removed. However, if the movable element 24, 24 of the switch 16 is now reversed to contact the first set of contacts 20, 20' the polarity of the voltage applied to the photoconductor 10 will be reversed and the decaying current through the load will be abruptly reduced to a very small value. The current flowing through the photoconductor 10 thus becomes extremely low and will be equal to a flow to which it would not reach even in minutes or hours if the radiant excitation were merely removed from the circuit,

In a light amplifier, the load circuit is a light emitting cell and alternating rather than direct voltage is applied to the circuit. Current will continue to flow through the poll and cause the image to remain after the excitation is removed if some means of reducing the photoconductor current to a low value is not provided.

As an example, a cadmium sulfide photoconductive powder placed across electrodes spaced .5 millimeter apart and 5 millimeters long will yield a current of about 200 microamperes when illuminated with a .5 foot candle of tungsten light with 400 volts D.C. applied across the electrodes. After the light excitation is removed it requires approximately 60 seconds for the current through the photoconductor to fall to .01 microampere. However, if immediately following the removal of the illumination the applied voltage is reversed in polarity the current through the the photoconductor will instantly assume a value of less than .001 microampere.

It has been found that the more intense is the illumination of the photoconductor by light or other radiation prior to erasure the lower will be the resultant current upon reversal of the polarity of the voltage. This same result will be observed if the operating voltage is made higher or if the time of illumination is lengthened. Thus, if a light amplifier panel has an image formed thereon that has bright areas and dark areas, a reversal of the voltage will erase the image, but the current flowing through the areas that had been brightly lit may be less than current flowing through the areas that had been less brightly lit. The erasing action thus may not be completely uniform over the surface of the amplifier panel and may result in a negative image of the erased picture being formed when a new image is projected on the panel. This condition of diiferent currents flowing after erasure may be eliminated by flooding the panel with light, as by closing the switch 21 to light the source 23, or X-ray radiation immediately prior to erasure. The light or X-rays flooded on the panel fixes the current immediately before erasure at the same value in all picture areas of the panel, so that after erasure the current flowing will be equal in all picture areas. Another manner in which to equalize the current flow after erasure is to flood the panel with a small amount of radiation after erasure to increase the current flow over the entire panel from a very low level to a higher value that is the same in all picture elements. This requires in a light amplifier that the current not be increased to an extent that would cause the electroluminescent material to emit light.

The manner in which this action may be specifically applied to a light amplifier device is illustrated in Figure 2. A light amplifying panel 39 consists of a transparent support panel 31, such as a glass sheet, on which a sheet of transparent conducting material 32, which may be a film of tin chloride, is aflixed. A layer of electroluminescent material 34 is fixed to the other side of the conducting material 32, and a current diffusing layer 36 is aflixed to the opposite face of the electroluminescent material 34. The photoconductive material or layer 38 is adjacent the current diffusing layer 30, and is grooved to provide ridges 39 for electrical connection to the photoconductor marterial 38. The current diffusing layer 36 may comprise cadmium sulfide which has been made conducting by first adding cadmium chloride and then heating the mixture to about 700 degrees Centigrade for 30 minutes, and the electroluminescent layer may comprise an electroluminescent phosphor such as zinc sulfide activated with copper and mixed with a suitable plastic such as ethyl cellulose.

In order to provide operating voltages to the amplifier panel, alternate ridges of the photoconductive layer 38 are connected through the common terminals 18, 18' of a double-pole, double-throw, switch 16. A first and second set of contacts 20, 20 and 2 2, 22, respectively, are

cross-connected as shown in Figure 1 and one contact 20 is connected through a first diode 40 to the secondary winding 42 of a transformer 44, the other terminal of the secondary winding 42 being connected to ground as a point of reference potential for the circuit. The remaining contact 20 is connected through a second diode 46 poled in the opposite direction to the first diode 40 to the secondary winding 42. An alternating voltage is applied to the primary winding 48 of the transformer 44 to provide operating voltage for the amplifier panel 30.

In order to describe the operation of the device, assume that the movable elements 24 of the switch 16 are connected to the first set of contacts 20, 20'. The various layers of the light amplifier panel 30 are thus in series with the voltage appearing across the secondary winding 42 of the transformer 44. The supply voltage is adjusted so that the magnitude of the voltage, with no light incident on the photoconductor 38, across the electroluminescent layer 34 is below the threshold value required to cause luminescence of the electroluminescent layer 34.

If now light is made incident upon the photoconductive layer 38 the conductivity of the irradiated photoconductor material is increased as a function of the intensity of the light. This action causes a corresponding increase in the voltage across the electroluminescent layer 34-, thereby providing a field of suflicient magnitude to energize the electroluminescent material and light will be emitted.

The fields that are applied to the electroluminescent material 34 and to the photoconductive material 38 are different in character. The use of the diodes 40 and 46, which are each connected to alternate ridges of the photoconductive material 38, provide that a pulsating direct voltage is applied to the ridges of the photoconductor layer 38 and an alternating field is applied to the electroluminescent layer 34. Alternate sets of ridges have a voltage applied thereto that is of opposite polarity and out of phase, that is, when one set of alternate ridges has voltage applied thereto, the other set has no voltage applied. This field distribution has been found to increase the sensitivity of the light amplifier.

If now the incident light is removed, the image formed on the light amplifier panel 30 will begin to decay. However, it'will decay at a relatively slow rate. As explained previously with respect to Figure l the current in the photoconductor 38 may take minutes to decay sulficiently to extinguish the light and a longer time to decay to a value sufficiently low that no after-image will be formed when new light is applied to the panel. Therefore, a substantial amount of time must elapse before a new image can be made incident on the light amplifier. Even though the previous image on the light amplifier is no longer visible a new light image may re-excite the old image if the current in the photoconductor has not reached a sufficiently low value.

In accordance with the invention, the current in the photoconductor may be abruptly reduced to a very low level equivalent to its level after many hours in the dark, by reversing the movable elements 24 of the double-pole, double-throw switch 16 to contact the second pair of contacts 22, 22'. This action reverses the field applied to the photoconductor 38 and will abruptly reduce the photoconductor current to its low current value. A new image may then be projected on the amplifier with little or no after-image as would normally remain with conventional decay. It will be appreciated, of course, that the reversal at the applied voltage may be accomplished by an electronic switch, and that the mechanical switch illustrated is shown for the sake of simplicity. The switching may be done continuously and reversed in fractions of a second, so that a moving image may be used on the light amplifier panel.

The erasing action may be improved in the manner previously described if, before the reversal of the switch 16, the photoconductor 38 is illuminated with strong light or X-rays over itsLentiresurface; asby closing the switch 21 to light thelamp'23'.

The invention may also .be applied to a storage light feed back panel. Each elemental area of the feed back,

panel may be composed of a'pair of photoconductor elements, one electroluminescent cell and diodes con,- nected as shown in Figure 3, to which reference is now made. The energizing circuitis the same as that shown with respect to Figure 2' and includes the transformer 44 the diodes 40 and 46 and the double-pole, doublethrow switch 16. Eachelemental area of the storage panel includes first and second photoconductor elements 50 and 52 connected serially 'between'the common contacts 18, 18 of a'double' pole, double-throw, switch 16,-

and an electroluminescent element 54'isconnected between the'junction of the photoconductor elements 50, 52 and the ground side'of the transformer secondary 42. The electroluminescent element 54 and the photo conductor elements 50, 52, are physically arranged so that light that is emitted by the electroluminescent element 54 is incidentupon, the photoconductor elements 50, 52, as feed back light to maintain the light output from the panel after the incident light which initiates the action is removed." This action results in a storage of the information that 'is applied to the element in the form of light or 'X-rays.

If it is desired to. removethe stored light the applied operating voltages may be removed. The change of re sistivity of the photoconductor, however, is independent of whether the circuit'is open or closed and the decay of its current carryin g capacity follows the manner previously described. The Current carrying capacity through the photoconductor elements 50and 52 does not decay to its low value'for some time after the removal of the operating voltages, therefore, voltagemay not be reapplied for ant-appreciable. time without re-exciting the previously stored information.

In accordance with the invention, the stored information may be immediately erased and the panel conditioned to receive entirelynew information by reversing the double-throw, double-pole, switch 16 to connect the movable elements 24, 24 to the second set of contacts 22, 22. This action immediately reduces the current in the photoconductors 5t}, 52 to very low values and the panel is then immediately in condition to receive new information without danger of re-exciting the previously stored information. As explained with reference to Figure 1, the erasing action is enhanced if the panel is flooded with strong light from the lamp 23 or X-rays immediately prior to the reversal of the double-pole, double-throw switch 16.

It has been noted that the change in resistivity and consequently the decay of current through a photoconductor material with direct voltage applied to the material is relatively slow. With direct voltage on the photoconductor, however, the sensitivity of a light amplifier device is quite high. If an alternating voltage is applied to the photoconductor the sensitivity of a light amplifierdevice is considerably lower, however, the rate of decay of the current through the photoconductor may be as high as times or more greater than with direct voltage applied to the photoconductor. Advantage is taken of this fact in the circuit shown in Figure 4, to which reference is now made and in which a light amplifier circuit includes a pair of photoconductor elements 50 and 52 connected in series with a pair of variable taps 56 and 58, each connected to one of a pair of batteries 60 and 62. The variable taps 56 and 58 are ganged together. An electroluminescent cell 54 is connected between the junction of the photoconductors 50 and 52 and ground for the system. An alternating voltage is applied to the terminals 64 and 66 to supply power to the devices. One terminal 66 is connected to ground and the other terminal is connected in common to a pair of fixed midtaps 65 and 67, each connected to one of the batteries 60 and 62. The func-.

tion of the batteries 60 and 62 is basically the same as the rectifiers 40 and 46 described with reference to Figures 2 and 3, that is, to provide a pulsating direct voltage for the photoconductors 50 and 52 and an alternating voltage to the electroluminescent cell 54-. The use of the batteries 60 and 62 rather than the diodes 40 and 46 and the switch 16 makes it possible to provide a more versatile erasing circuit.

The operation of the circuit is fundamentally the same as that of Figures 2 and 3. A positive bias on photoconductor 50 and a negative bias on photoconductor 52 equal to the peak value of the alternating voltage is pro vided by the batteries 60 and 62. The photoconductors 50 and 52 will thus be biased so that the entire alternating voltage will appear as a pulsating direct voltage to the photoconductors 50 and 52. The voltage across the electroluminescent cell 54 is merely the alternating voltage applied, since the electroluminescent cell is not biased in any way. In order to erase, it is only necessa'ry to reverse the polarity of the biasing voltages ap plied to the photoconductors 50 and 52. This may be accomplished by moving the variable taps 56 and 58 on the batteries 60 and 62 so that a positive bias is applied to the photoconductor 52 and a negative bias to the photoconductor 50. The complete reversal of the bias will reverse the polarity of pulsating direct voltage applied ,to the photoconductors 50 and 52and instantly reduce the current therethrough to a low value. However, if an abrupt erasure is not desired the variable contacts 56 and 58 may be moved to the center tap points 65 and 67 of the batteries 60 and 62 and only A.C. will be applied to the photoconductors 50 and 52. As previously mentioned, this will cause an increase in the rate of decay of the current through the photoconductors 50 and 52. If desired the variable taps 56 and 58 may be placed at any intermediate point in the batteries 60 and 62 to provide a selectable rate of decay. In order to maintain the sensitivity of the amplifier at a constant value it may also be necessary to vary the amplitude of the alternating voltage applied at terminals 64 and 66 simultaneously with the variation of the bias voltages. Thus the decay characteristic of the current through the photoconductors may not only be abruptly controlled but the rate of decay may be controlled by controlling the bias voltage applied to the photoconductor elements.

A controlling circuit for a photoconductive material in accordance with the invention provides means for rapidly removing the decaying current through the photoconductor. Such circuits may find wide use with devices such as light amplifiers, insuring rapid erasing of the light image and enabling their immediate use for the reception of a new image without causing an after image resulting from the erased image.

I claim:

1. In combination with a light amplifier device, said device including a body of photoconductive material having a variable impedance characteristic in response to radiant energy excitation serially connected with a body of electroluminescent material emitting light in response to an applied electric field, means for applying a substantially unidirectional electric field to said photoconductive body, means for applying an alternating field to said electroluminescent body, means for exciting said photoconductive body with radiant energy tocause light to be emitted from said electroluminescent body, and means for reversing the polarity of the electric field applied to said photoconductive body to erase the light emitted by said electroluminescent body.

2. An image erasing circuit for a light amplifier device, said device including a body of photoconductive material having a variable impedance characteristic in response to radiant energy excitation and a body of electroluminescent material emitting light in response to an ap- 7 plied voltage, comprising in combination a source of alternating supply voltage, means for applying from said source a substantially unidirectional voltage to said photoconductive material and an alternating voltage to said electroluminescent material, means for exciting said photoconductive material with radiant energy to reduce the conductivity thereof and increase the voltage applied to said electroluminescent material and cause light to be emitted therefrom, means for removing the radiant excitation of said photoconductive material to cause the current therethrough to decay, and means for varying the voltage applied to said photoconductive material to control the decay of the current therethrough and erase the light emitted from said electroluminescent material.

3. An image erasing circuit for a light amplifier comprising in combination, an electroluminescent device including a layer of adjacent elements of a photoconductive material having a variable impedance characteristic in response to radiant energy excitation, an electroluminescent layer adjacent to said layer on one side thereof, a conductive layer coextensive with said electroluminescent layer, said electroluminescent layer being supported between said conductive member and said elements whereby said elements and said electroluminescent layer are electrically in series, means for applying an alternating voltage of opposite phase between said conductive layer and alternate elements of said photoconductive layer and of opposite phase between said conductive layer and intermediate elements of said photoconductive layer for applying a unidirectional field across individual elements and an alternating field across said electroluminescent layer, and means for varying the uni directional field applied to said photoconductive layer to erase an image on said device.

4. An image erasing circuit for a light amplifier storage device, said device including a body of photoconductive material having a variable impedance characteristic in response to radiant energy excitation in electrical and optical contact with a body of electroluminescent material emitting light in response to an applied electric field, comprising in combination a source of alternating supply voltage, means for applying from said source a substantially unidirectional voltage to said photoconductive material and an alternating voltage to said electroluminescent material, means for exciting said photoconductive material with radiant energy to vary the conductivity thereof and increase the voltage applied to said electroluminescent material whereby light is emitted from said electroluminescent body, and means for varying the voltage applied to said photoconductive material to cause decay of current therethrough and erase the light emitted from said electroluminescent material.

5. A light amplifier comprising incombination a light amplifier device including a body of electroluminescent material having a pair of photoconductive elements in contact therewith, a first source of direct voltage poled in a predetermined direction and connected to one of said photoconductive elements, a second source of direct voltage poled in the opposite direction to said first source and connected to the other of said photoconductive elements, a conductive member in contact with said body of electroluminescent material, said electroluminescent material being supported between said conductive member and said photoconductive elements whereby said elements and said body of electroluminescent material are electrically in series, means providing a source of alternating voltage connected between said conductive member and said direct voltage sources for applying a unidirectional voltage to said photoconductive elements and an alternating voltage to said body of electroluminescent material, and means for varying the voltage of said first and second direct voltage sources to efiect control of the decay of current through said photoconductive elements and erasure of the light emitted by said amplifier.

References Cited in the file of this patent UNITED STATES PATENTS 2,743,430 Schultz et al.- Apr. 24, 1956 2,773,992 Ullery Dec. 11, 1956 2,897,399 Garwin et al. a July 28, 1959 

